If a vertebral body is so severely damaged through disease or injury that it can no longer satisfy its supporting function between the two adjacent vertebral bodies, it can be partially replaced by a known prosthesis (DE-A-4109941, FIG. 8). This prosthesis comprises an upper contact plate for connection to the upper adjacent vertebral body, a lower contact plate for connection to the lower adjacent vertebral body, and, between them, a bridging part which bridges the vertebral body that is to be partially replaced. If the vertebral body to be partially replaced is completely preserved only on the vertebral arch side, a bone cavity is created on its front face and the bridging part is inserted into said bone cavity. For firm connection to the vertebral body to be partially replaced, the bridging part has laterally protruding webs which contain an oblong hole for receiving a securing screw. The securing of the prosthesis on the vertebral body to be partially replaced determines, in addition to the facet articulations, the position of said vertebral body with respect to the adjacent vertebral bodies. Only when its front faces, intended for the securing webs to bear on, are worked in such a way that the vertebra, after connection to the webs, can maintain its natural position defined by the facet articulations, is there any prospect of the prosthesis fitting so exactly that the facet articulations match in exactly the natural manner and no pain caused by movement arises. Such precise working is difficult to achieve in the confined operating site, especially as the position of the prosthesis is influenced not only by the surfaces of the bone cavity, but also by the front face and the screw holes. Inaccuracies in the production of each of these surfaces impair the exactness of the prosthesis position and may therefore lead to pain caused by movement.
A prosthesis for partial replacement of a vertebral body is known (U.S. Pat. No. 4,892,545) whose bridging part is anchored only on the two vertebral bodies adjacent to the vertebral body to be partially replaced, and which for this purpose has projections which are driven upward and downward into the adjacent vertebral bodies after the prosthesis has been inserted. These are intended to be immobilized in relation to the bridged vertebral body. No hinge is present. Since there is therefore no movement between adjacent vertebral bodies, there is also no fear of pain caused by movement resulting from imprecise positioning of the prosthesis. This known prosthesis could not therefore teach the skilled person that, by using retractable and advanceable anchoring projections arranged at another location, pain caused by movement could be avoided in prostheses equipped with a hinge.
Mechanisms for moving the anchoring projections are known or have been the subject of previous applications (EP-A-951879, EP-A-1104665, U.S. Pat. No. 4,636,217, U.S. Pat. No. 5,658,335, US-A-2002/0165613, US-A-2004/0088054, WO2004/080356). The drive means can be configured in such a way that, by being turned, it can bring about the movement of the anchoring projections. It can, for example, be configured as an eccentric cam or as a rotary cam which, in the position of rotation corresponding to the implantation position of the projections, protrudes less far toward the projections than it does in the position of rotation corresponding to the anchoring position. A particularly advantageous embodiment of the drive means is a pinion which meshes in toothed racks connected to the anchoring projections. This has the advantage that the travel covered by the projections between the implantation position and the anchoring position is not limited to the cam height of the drive means.